well I wouldn't class Maths as utilities - although it is in a way a collection of utilities - it's often better seen as more of a complex modulation source / swiss army knife module and fantastic learning tool for modular patching and thinking in general - see the "maths illustratated supplement" for ideas
the mixer and mults and vcas(?) though definitely are indispensable utilities - as I am sure you will find the attenuversion and offsets in Maths so useful that you will want them as separate modules (shades, 3*mia etc) very soon - so that you can use Maths for more interesting things
I'd seriously consider adding a quad cascading vca such as veils or the equivalent from intellijel or happy nerding these are near fundamental to synthesis - for processing both audio and modulation signals
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities